This invention relates to a digital radio receiver for receiving an electromagnetic wave from an artificial satellite (that may be called a “satellite wave”) or an electromagnetic wave on the ground (that may be called a “terrestrial wave”) to listen in a digital radio broadcasting and, in particular, to a loop antenna for mainly use in the digital radio receiver.
In recent years, a digital radio receiver, which receives the satellite wave or the terrestrial wave to listen in the digital radio broadcasting, has been developed and is put to practical use in the United States of America. The digital radio receiver is mounted on a mobile station such as an automobile and can receive an electromagnetic wave having a frequency of about 2 gigahertz (GHz) to listen in a radio broadcasting. That is, the digital radio receiver is a radio receiver which can listen in a mobile broadcasting. In addition, the terrestrial wave is an electromagnetic wave in which a signal where the satellite wave is received in an earth station is frequently shifted a little.
In order to receive such an electromagnetic wave having the frequency of about 2 GHz, it is necessary to set up an antenna outside the automobile. Although a variety of antennas having such various structures as planer-type (plane-type), cylindrical-type or the like have been proposed, the antennas of cylindrical-type are generally used rather than those of planer-type (plane-type). This is because a wider directivity is achieved by forming the antenna into a cylindrical shape.
A loop antenna is known in the art as one of the antennas of the cylindrical-type. The loop antenna has structure where one antenna lead member is wound around a peripheral surface of a hollow or solid cylindrical (which is collectively called “cylindrical”) member in a loop fashion, namely, is an antenna having the form of a loop. It is known in the art that the loop antenna acts as an antenna having a directivity in a longitudinal direction of the cylindrical body axis if the antenna lead member has an all around length which is selected to about one wavelength. This is because the antenna lead member has a sinusoidal distribution of a current.
A conventional loop antenna is practically used as the antenna of the digital radio receiver. However, inasmuch as such structure has the directivity for a longitudinal direction to axis of the cylindrical body, it is not possible that the loop antenna sufficiently catches the terrestrial wave nearly in a horizontal plane of a vertical direction to the axis of the cylindrical body.
For example, as Illustrated in FIG. 1, a conventional loop antenna comprises a cylindrical body 11 and a loop antenna element 10. The cylindrical body 11 is formed by rounding a flexible insulator film member around a central axis in a cylindrical fashion. The loop antenna element 10 is made of conductor, and formed on the cylindrical body 11 along the peripheral surface around the central axis in a loop fashion. The loop antenna 1 further comprises four feeders 12 formed on the peripheral surface of the cylindrical body 11 to feed to the loop antenna element 10 at four points. One end in each of the four feeders 12 is directly connected with the loop antenna element 10. The other end in each of the four feeders 12 is connected with an antenna device circuit mounted on a circuit board 13.
That is, the conventional loop antenna 1 of the type described is formed by mounting the cylindrical body 11 with the loop antenna element 10 and four feeders 12 on the circuit board 13 and by connecting and fixing the other end of each of the four feeders 12 with the circuit board 13. Here, the number of feeders is shown as four which is an example and may be settled by any number.
Referring now to FIG. 2, description will be made of antenna radiation characteristic of the loop antenna having the directivity in a longitudinal direction of the cylindrical body axis.
FIG. 2 shows an antenna radiation pattern of LHCP (left-hand circular polarization wave) gains in frequency 2 GHz band in case of measuring the gain of the loop antenna element having following sizes in the shape shown in FIG. 1 and placed on a ground plate. That is, FIG. 2 shows a pattern of the gains for the left-hand and right-hand circular polarization waves in a vertical plane including the central axis of the cylindrical body, setting zero degree of a horizontal plane of the loop antenna element crossing at right angles with the horizontal plane and 90 degree of a top of the axis.
The data shown in FIG. 2 are obtained by following conditions. The cylindrical body with an outside diameter of 20 mm and a height of 20 mm is placed on a center of a surface of a ground plate with a diameter of 1 m, placing commonly a central axis of the cylindrical body and a vertical axis of the ground plate. And the loop antenna element is provided along the peripheral surface around the top of the cylindrical body.
As shown in FIG. 2, in the loop antenna having structure described above, a gain obtained for each angle from −20 degrees to +20 degrees on either side is very small comparing with a gain for an angle of elevation on +20 degrees or more. This means a defect that, on the level surface formed by the loop antenna element, the gain is not good enough obtained from electromagnetic wave arriving within range of the low angle of elevation.
Such a defect is very large obstacle for using the above-described loop antenna to the digital radio receiver loaded onto an automobile. The central axis of the loop antenna loaded onto an automobile is placed to vertical direction of the level surface in order to avoid any fluctuation of receiving level corresponding to change the direction of the automobile. On the other hand, it is sure that electromagnetic waves become weak because of obstacle on the ground for either the satellite wave or the terrestrial wave within the range of low angle of elevation.
Accordingly, the conventional loop antenna is disadvantageous in that the gain needs any increase in the range of low angle of elevation against the level surface.